Head mounted display device (hereinafter “HMD”) is a display device which can be worn over a head. HMD generally employs a “near-eye optical system” to project a display source of a display screen into eyes of a user. HMD is divided into an immersion and a penetration. For an immersive device, it allows the user to fully immersing the display content provided by the device, which isolates the user from a contact with a real world.
In order to solve the problem that the immersive HMD isolates the user from the contact with the real world, a current general way is to set an external camera on the HMD. The user can switch the external camera as an output display source and display external scenes captured by the camera through the HMD. However, the switch of the external camera may generate some certain time delay and some location offset, which leads to a difference between the external scene displayed by the HMD and the real scene, resulting unreality.